1. Field of the Invention
The present invention relates to a method of controlling condition of an assembled battery mounted in a self-generation electric vehicle such as a hybrid vehicle.
2. Description of the Related Art
An assembled battery mounted in an electric vehicle is discharged mostly when the vehicle accelerates and charged when the vehicle runs at a constant speed or decelerates. Therefore, it is necessary to maintain the condition of such a battery at a intermediate level between fully-charged level and fully discharged level.
In order to maintain the battery condition at the intermediate level, it is necessary to frequently detect the remaining capacity. In the past, the remaining capacity was estimated by summing up battery charging currents and battery discharging currents.
JP-A-10-51906 discloses such a method of controlling, as the battery condition, an SOC (State of Charge) ratio which represents the remaining capacity. Data related to terminal voltage and current of an assembled battery at a maximum normal SOC ratio and a minimum normal SOC ratio are stored in a map and are compared with an estimated SOC ratio calculated from currently detected terminal voltage and charge or discharge current of the battery.
If the estimated SOC ratio approaches the maximum normal SOC ratio, the battery is discharged. On the other hand, the battery is charged if the estimated SOC ratio approaches the minimum normal SOC ratio. This control is repeated frequently.
The more errors are accumulated in the estimated SOC ratio as more frequently the above steps are repeated. This may cause improper control of the battery condition.
In the disclosed method, the battery is always controlled so that the SOC ratio stays either at the maximum normal SOC ratio or at the minimum normal SOC ratio. If the battery is controlled at the minimum normal SOC ratio, it is difficult to supply driving energy from the battery to the wheels. On the other hand, if the battery is controlled at the maximum normal SOC ratio, it is difficult to charge the battery with the electric power regenerated from the driving energy of the wheels.
Further, because the assembled battery is comprised of a plurality of battery cells of different remaining capacities (or different SOC ratios) and deterioration speeds, it is always possible that any one of the battery cells is over-discharged or deteriorating.